Cancer is a leading cause of death in the U.S. The incidence of cancer within the population varies with respect to age of onset, tumor type and tissue specificity. The underlying mechanism of cancer manifestation is believed to be generated by multiple genetic alterations. These genomic changes in the development of cancer could involve insertion, deletion, duplication, amplification, point mutation and recombination of DNA sequences. Such alterations can be inherited in the germ line or acquired in somatic cells. Individuals with inherited cancer syndromes possess an altered gene and therefore they require one less genetic event for cancer to develop. We have shown in our previous studies that some high risk cancer families with histories of cancer consistent with Li-Fraumeni syndrome, LFS, carry germline mutations in the tumor suppressor gene p53. These mutant p53 genes alter the behavior of the normal cells of these patients causing them to rapidly develop some abnormal properties including altered morphology, chromosomal instability, escape from growth controls associated with cellular senescence and susceptibility to tumorigenic transformation by a ras oncogene. These are properties often associated with the development of cancer. We will mechanistically characterize how p53 functions to generate the genomic instability which ultimately results in the severe aneuploidy and immortalization observed in LFS fibroblasts. We will test p53 genes with various inherited mutations as well as functional domains of p53 for the ability to induce chromosomal instability and morphological changes, escape from senescence and changes in rates of genetic rearrangements and their mechanisms. Some but not all of the LFS fibroblasts that have escaped senescence became tumorigenic in nude mice after transfection of an activated ras oncogene. It appears that an oncogene suppressing function in these cells is lost during this time in culture. To understand how p53 participates in carcinogenesis, we will further characterize this oncogene suppressing function in LFS fibroblasts.